FIG. 1 shows a block diagram of a phase-comparison monopulse antenna system 100. Monopulse antenna system 100 includes an antenna 102 and a monopulse feed or comparator 104 that operate to determine the angle of an incoming radio frequency (RF) signal relative to the boresight of antenna 102. As shown in FIG. 1, antenna 102 is divided into four quadrants 111-114. Monopulse comparator 104 detects the relative phase differences of the incoming RF signal as received in each quadrant, and provides signals indicative of the relative phase differences to a receiver 106. Typically, the monopulse comparator provides an azimuth phase difference signal (.DELTA.AZ), an elevation phase difference signal (.DELTA.EL), and a sum signal (.SIGMA.). Receiver 106 uses the .DELTA.AZ and .DELTA.EL signals to determine the elevation and azimuth angles of the incoming RF signal relative to the boresight of antenna 102. The .SIGMA. signal represents the incoming RF signal received by antenna 102 as a whole (i.e, a portion of the RF signal as received by all of the quadrants is summed and provided as the .SIGMA. signal). The signals from the monopulse comparator are typically transmitted to the receiver by waveguides. Receiver 106 typically includes waveguide-to-stripline or waveguide-to-coaxial transitions to transition the waveguide propagated signals to stripline or coaxial signals.
FIG. 2 diagramatically illustrates antenna system 100 providing the .DELTA.EL signal from the incoming RF signal. In this conventional monopulse antenna system, monopulse comparator 104 feeds a portion of the incoming RF signal as received by the upper half 102A (i.e., sums the incoming RF signals as received by quadrants 111 and 112 in FIG. 1) of antenna 102 along a channel 200. Concurrently, monopulse comparator 104 feeds a portion of the incoming RF signal as received by the lower half 102B (i.e., sums the incoming RF signals as received by quadrants 113 and 114 in FIG. 1) of antenna 102 along a channel 202. If the incoming RF signal is at an elevation angle of .theta. from the boresight, then the incoming RF signal must travel an extra distance d to reach the lower half 102B. As a result, there is a difference in phase between the RF signal as received by upper half 102A and the RF signal as received by lower half 102B. In addition, monopulse comparator 104 introduces a 180.degree. phase shift into the lower half signal at channel 202 and then sums the signals propagated in channels 200 and 202 to provide the .DELTA.EL signal. Consequently, monopulse comparator 104 in effect subtracts the RF signal as received from lower half 102B from the RF signal as received by the upper half 102A.
For example, when the incoming RF signal is aligned in elevation with the antenna's boresight, the summed signal from the upper two quadrants has substantially the same phase as the summed signal from the lower quadrants and, thus, the 180.degree. phase shifted signal from the lower two quadrants substantially cancels the summed signal from the upper two quadrants. Consequently, the power of the .DELTA.EL signal is substantially zero. However, if the incoming RF signal is at an elevation angle .theta. with the boresight, then the signals from the upper and lower halves do not cancel each other, and the .DELTA.EL signal will have some power. The ratio of the power of the .DELTA.EL signal to the power of the .SIGMA. signal is proportional to elevation angle .theta..
Similarly, monopulse comparator 104 simultaneously provides the .DELTA.AZ signal by phase shifting a portion of the incoming RF signal as received by the left half of antenna 102 (i.e., summing the incoming RF signal as received by quadrants 111 and 114 in FIG. 1) and summing this phase shifted signal with a portion of the incoming RF signal as received by the right half of antenna 102 (i.e., summing the incoming RF signal as received by quadrants 112 and 113 in FIG. 1).
FIG. 3 shows a conventional monopulse comparator 300 with a horn antenna 323. The boresight of horn antenna 323 is indicated by a broken line 326. As can be seen, monopulse comparator 300 has several waveguide feeds, a substantial portion of which lie in a plane parallel to or containing the boresight of horn antenna 323, which is referred to herein as a nonplanar-configuration. Monopulse comparators with a nonplanar-configuration are relatively large, thereby increasing the size and thickness of the monopulse antenna system. Further, monopulse comparators with a nonplanar-configuration are relatively costly to fabricate and may be impractical or impossible to fabricate for applications above 75 GHz.
In applications that require small size and/or thickness, a monopulse comparator with a configuration having substantially all of the monopulse comparator's waveguides lying within a single plane perpendicular to the boresight of the antenna can be used (i.e., a planar-configuration). However, conventional monopulse comparators with a planar-configuration are relatively expensive and operate at a narrow bandwidth. For example, one 35 GHz planar monopulse comparator currently available uses a split block waveguide assembly with 90.degree. short slot hybrids and 90.degree. phase-shifters. At 35 GHz, the short slot hybrids have narrow walls that make machining difficult and expensive. Further, this comparator uses relatively long interconnecting waveguides that increase the area of the monopulse comparator and make matching the waveguide lengths more difficult. Moreover, the 90.degree. hybrids and 90.degree. phase-shifters used are generally accurate for only a single frequency, thereby decreasing the bandwidth of the monopulse comparator. This monopulse comparator currently costs approximately $5,000.